Kepler Mission Data Reveal First Non-Transiting Planet Candidates

HOUSTON — Researchers using observations gathered by NASA’s Kepler Telescope claim the first discovery of a non-transiting exoplanet, revealed through gravitational perturbations called transit timing variations, or TTVs.

The TTV technique, first proposed as a strategy for finding Earth-like exoplanets seven years ago, could reveal evidence of more extrasolar planets hidden away in existing Kepler observation data.

The discovery of an unseen Planet C, circling the star Kepler Object of Interest-872, was reported May 10 in the journal Science by David Nesvorny, staff scientist at the Southwest Research Institute of Boulder, Colo., and five colleagues.

KOI-872, a Sun-like star 2,800 light-years away in the direction of the Milky Way’s center, first came under scrutiny by Nesvorny’s team after the public release of Kepler data in January 2012 as part of an exo-moon search project.

The target star was selected at the time because of Planet B, a transiting Jupiter-sized planet candidate discovered by the Kepler team.

Planet B’s 33.6-day orbital period displayed visual transit anomalies and other dynamics suggesting the presence of a possible moon. Further analysis and modeling by Nesvorny and his colleagues revealed a 2-hr. variation in KOI-872b’s transit time.

“To put this in context, if a bullet train arrives in a station two hours late, there must be a very good reason for that,” Nesvorny says. “The trick is to find what it is.”

The modeling suggested the presence of Planet C, a Saturn-sized body with a 57-day orbit period that barely avoids transiting the host star in Kepler’s visual plane, rather than a moon, as the source of the perturbation, Nesvorny reports.

The TTV technique was used by the French mathematician Urbain Le Verrier to predict the presence of Neptune well before the planet’s 1846 discovery. The prediction was based on small variations in the motion of Uranus.

Further modeling by Nesvorny and his colleagues suggests KOI-872 also hosts a third planet, a super Earth body, currently designated KOI-872.03, that orbits every seven days.

Kepler, whose mission was recently extended through fiscal 2016, was launched in March 2009 to search for evidence of Earth-like planets in the habitable zones of 150,000 Milky Way stars.

The telescope’s optics record small dips in stellar brightness that correspond to the transit of planets as the primary detection technique.

As the initial phase of the Kepler mission came to a close earlier this year, the transit candidate planet count stood at 2,321, including 200 Earth-sized planetary objects. Roughly 60 of the candidates have cleared a stringent confirmation process.

The count on each of those fronts, including Earth analogs, has the potential to climb as researchers apply TTV modeling analysis techniques to Kepler mission data, Nesvorny said in an email exchange.

“The TTV method was suggested back in 2005 as a detection technique for Earth-mass planets,” Nesvorny notes. “It certainly has the potential for finding small planets, but there are some special conditions that need to be met.”

Those include the wide existence of multi-planet systems with at least one transiting planet and favorable timing.